Barrier Height Modification of n-InP Using a Silver Nanoparticles Loaded
Graphene Oxide as an Interlayer in a Wide Temperature Range
Article in Journal of Electronic Materials · February 2019 DOI: 10.1007/s11664-019-07088-8 CITATIONS 0 READS 116 6 authors, including: Baltakesmez Ali Ardahan University 6PUBLICATIONS 3CITATIONS SEE PROFILE Züleyha Kudaş Ataturk University 2PUBLICATIONS 3CITATIONS SEE PROFILE Duygu Ekinci Ataturk University 32PUBLICATIONS 324CITATIONS SEE PROFILE
All content following this page was uploaded by Betül Güzeldir on 04 March 2019.
and M. SAG˘ LAM2
1.—Department of Electricity and Energy, Technical Scientific Vocational School, Ardahan University, 75000 Ardahan, Turkey. 2.—Department of Physics, Faculty of Sciences, Atatu¨rk University, 25240 Erzurum, Turkey. 3.—Department of Chemistry, Faculty of Sciences, Ataturk University, 25240 Erzurum, Turkey. 4.—e-mail: bguzeldir@atauni.edu.tr
Mercaptoundecanoic acid capped-Ag nanoparticles (MUA-AgNPs) assembled on graphene oxide (GO), namely MUA-AgNPs-GO nanocomposite, was used for enhancing current–voltage (I–V) activity and stability of n-lnP based heterojunction devices. The structural, morphological and optical properties of the MUA-AgNPs-GO nanocomposite were examined by Raman spectroscopy, UV–Vis spectroscopy, transmission electron microscopy and scanning electron microscopy measurements. Besides, the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction was fabricated, and working performance of the heterojunction was investigated in the temperature range of 80–320 K by steps of 20 K. The heterojunction created by the MUA-AgNPs-GO nanocomposite showed im-proved working performance such as better I–V characteristics, great stability and better rectifying ratio than that of our reference junction. The ideality factor and barrier height values of the junction formed with MUA-AgNPs-GO layer were found to be 1.07 eV and 0.630 eV, respectively. The experimental value of the Richardson constant was determined to be 3.82 A/cm2 K2 in the 80–160 K temperature range and to be 6.55 A/cm2K2in the 160–320 K tem-perature range. The results showed that the MUA-AgNPs-GO nanocomposite is a favorable candidate to provide modification of barrier height and to im-prove characteristic parameters for applications of the heterojunction devices. Key words: Graphene oxide, nanoparticle, heterojunction, current–voltage
measurement
INTRODUCTION
The GO is a two dimensional (2D) material that is derived from the parent graphene and has a strongly hydrophilic feature due to the large amount of oxygen containing bonds in its edges and defective sites. The oxygen atoms are covalently bonded to carbon atoms, converting them from the sp2 hybridized state in the parent graphene to the sp3 hybridized state.1 Moreover, the functional
groups in the GO could be easily reduced2,3 and it can used to build alternative nanocomposites such as Au, Ag, Pd and Fe3O4.4–6
Recently, reported results indicate that the nanocomposite of the GO with Ag or Au nanoparticles (NPs) are more active materials than individual parts of these materials due to the superior energy transfer between metal NPs and GO film. Until now, only a few reports have been presented about the hetero-junction characteristic parametres of GO-metal NPs.7–10Therefore, a simple and suitable synthesis method that allow it to be uniformly distributed components is very important for (opto)electronic device applications, particularly for n-InP based. The
most prevalent synthesis methods of combination of the graphene and the Ag NPs are Hummer’s and Offeman method. The Hummer’s method uses potas-sium permanganate (KMnO4) as on oxidant in
con-centrated sulfuric acid medium. Since the reaction immediately takes place, the method is more effective compared to other methods.11
The n-lnP based heterojunction structures have crucial significance for optoelectronic device appli-cations such as Schottky solar cells, photodiodes, LASER, high-speed digital and high frequency microwave devices.12–15 Therefore, they have recently become an attractive candidate in optical fiber communication systems based on low-loss and low-dispersion optical fibers. Furthermore, they are also suitable for the efficient operation of various transport devices such as Gunn diodes and high-speed heterojunction transistors.16–18
Herein, we have primarily studied characteriza-tion of the MUA-AgNPs-GO nanocomposites by Raman spectroscopy, TEM, SEM and UV–Vis spec-troscopy. After good characterization of the MUA-AgNPs-GO nanocomposites, it was used as an interlayer onto the n-InP wafer. The obtained heterojunction was investigated to calculate barrier height and characteristic parameters of the hetero-junction. In this process, the forward bias I–V–T characteristics of the heterojunction were system-atically analyzed over the temperature range 80– 320 K by steps of 20 K.
EXPERIMENTAL PROCEDURE Chemicals
Silver nitrate (AgNO3), 11-mercaptoundecanoic
acid (MUA), sodium borohydride (NaBH4) and
graphite were purchased from Sigma-Aldrich. All other reagents and solvents were also commercially purchased and used without purification.
Synthesis of the MUA-AgNPs
The silver NPs were synthesized according to a procedure described in the literature.19 Briefly, AgNO3(3 9 102M) and MUA (1.3 9 104M) were
dissolved in 10 mL of ethanol under stirring at room temperature. Then, a saturated solution of NaBH4
in ethanol was added to this solution drop by drop, and the mixture was stirred at room temperature for 120 min. Afterwards, the unbound thiol was removed by a centrifuge and the NPs were washed three times with a mixture of the hexane and the ethanol.
Synthesis of the MUA-AgNPs-GO Nanocomposite
The GO was synthesized using graphite powder by modified Hummer’s method.20 Firstly, graphite powder (10 g) was gradually added into a hot solution of concentrated H2SO4 (50 mL) containing
K2S2O8(5 g) and P2O5 (5 g). The mixture was kept
at 80°C for 6 h, and then the resulting dark blue mixture was slowly cooled to room temperature. The mixture was diluted and filtered. The filtered pro-duct was dried overnight at 60°C. After this preox-idizitation process, the graphite powder (1 g) was put into 50 mL of concentrated H2SO4 at 0°C, then
NaNO3(1 g), and KMnO4(6 g) were slowly added to
the mixture for 60 min to avoid a sudden increase in temperature. The solution was stirred at 35°C for 3 h, followed by adding 50 mL of distilled water. After being stirred for 15 min, the reaction was terminated by addition of a large amount of water (500 mL) and 30% H2O2 solution (8 mL). The
pro-duct was filtered and washed with HCl and water. Finally, the resulting purified GO powders were collected by centrifugation and dried at 60°C for 36 h in an air-dry oven. GO (5 mg) was firstly dispersed in ethanol (5 mL) and was sonicated for 2 h. Then, 5 mg of AgNPs was dissolved in 5 mL of ethanol and next was added to the GO suspension dropwise. The obtained mixture was sonicated for 3 h to form homogeneous solution. The MUA-AgNPs/GO composite was collected by centrifuga-tion (8000 rpm for 15 min) and redispersed in ethanol (5 mL). Figure1a shows formation mecha-nism of the AgNPs-GO nanocomposite.
Preparation of Heterojunction Device
The n-InP (100) wafers were used as a substrate. The n-lnP wafer was cleaned in optimal organic solutions (trichloroethylene, acetone and methanol for respectively 15 min), and then kept in the acid solution of (H2SO4:H2O2:H2O = 3:1:1) for 1 min and
(HF:H2O = 1:10) for 1 min, respectively. Between
each polishing step, the wafers were rinsed in deionized (DI) water and dried with nitrogen gas, respectively. In this heterojunction, n-InP/Au-Ge junction has an ohmic character. For ohmic contact having minimized contact resistivity, Au-Ge alloy was evaporated onto the n-lnP and annealed at 465°C under N2 atmosphere for 5 min. The
MUA-AgNPs-GO nanocomposite was coated onto the other face of the n-InP wafer by static spin coating technique at 1000 rpm for 1 min. After the coating process, the MUA-AgNPs-GO film was annealed at 100°C for 10 min in ambient air. To determine the junction area and to perform the electrical mea-surement, Ag metal was evaporated onto the AgNPs-GO at 1.33 9 106 kPa. The contact area was measured to be 7.85 9 103cm2. The Ag/ AgNPs-GO junction has a low resistance ohmic contact due to lower work function of Ag metal compared to work function of GO. On the other hand, AgNPs-GO/n-InP heterojunction has a recti-fying behavior. Figure1b represents device struc-ture of the heterojunction diode.
Characterization Methods
The Raman spectra were taken with a WITec Alpha300 Micro Raman instrument. The UV–Vis
electronic absorption spectra were taken with a Shimadzu UV-3600 Plus UV–Vis–NIR instrument. The SEM and TEM images of MUA-AgNPs-GO nanocomposites were recorded by a FEI Technai G2 Spirit BiO(TWIN) instrument working at 120 kV and a Zeiss Sigma 300 field emission SEM instru-ment, respectively. The I–V measurements of the junction were carried out by using a Leybold Heraeus closed-cycle helium cryostat with KEITH-LEY 487 Picoammeter/Voltage Source in the tem-perature range of 80–320 K.
Additional to the measurements, carrier concen-tration of the sides of heterojunction should be evaluated in order to accurately estimate transport processes in the heterojunction. Although carrier concentration values of n-lnP is 2–8 9 1018 cm3, value of AgNPs/GO could not be determined due to a measurment system not available in our present systems.
RESULTS AND DISCUSSION Characterization of the MUA-AgNPs-GO Nanocomposite
Figure2shows the Raman spectra of the GO film coated by the spin coating on glass, the MUA-AgNPs-GO nanocomposite films coated by the spin coating on glass and on n-InP substrates. The Raman spectroscopy is used to obtain reliable information about the vibration of the atoms and the chemical composition of material. The spectra show D and G characteristic peaks located at 1331 cm1and 1595 cm1, respectively. The D peak is assigned to the breathing mode of k-point phonons of A1g symmetry, while the G peak is
ascribed to the plane vibrations with E2gsymmetry
that is sensitive to the configuration of sp2sites.21–23 In Fig.2, it can be seen that the intensity of D and G characteristic peaks is increased with loading AgNPs. Moreover, the peak intensities increased when the n-lnP is used as a substrate.
It is well known that the surface-enhanced Raman scattering (SERS) occurs with an
electromagnetic and chemical enhancement. The electromagnetic enhancement may be caused by an increment about 1012 while the chemical enhance-ment is responsible for a few orders of magnitude. In the spectra, the D and G Raman peak intensities of the MUA-AgNPs-GO nanocomposite film are stronger than that of the GO film in a few order of magnitude and the peak positions are not changed. Therefore, it can be attributed that the increased signal intensity is mainly due to chemical enhance-ment in this study. Thus, the surface of the GO film having many functional groups may act as the positioning center for chemical interaction of the AgNPs. It can be claimed that the MUA-AgNPs-GO nanocomposite can have a charge transfer complex due to the chemical SERS.
The UV–Vis spectroscopy is one of the most practical and suitable instruments for the charac-terization of metal NPs such as Ag, Au and Cu2O. A
series of absorbance spectra of the solutions are given in Fig.3a. The MUA-AgNPs, the GO and the MUA-AgNPs-GO nanocomposite were confirmed by the UV–visible spectra. Additionally, the MUA-AgNPs-GO nanocomposite thin film onto the glass substrate was analyzed by the UV–Vis measure-ment, as seen in Fig.3b. Besides, Fig.3a and b showed that the two peaks at 400 nm and 450 nm were consistent with surface plasmon resonance (SPR) phenomena of the AgNPs formulation.24,25It is well known that the SPR depend on metallic NPs size and allows strong coupling between the electron resonances with the light coming simultaneously on NPs.
The TEM images taken at different magnifica-tions of the MUA-AgNPs-GO nanocomposite are shown in Fig.4a and b. As can be seen in Fig.4b, large size and different forms of the AgNPs well distribute into the GO and it has been caused the nature and amount of present oxidized defects present on the GO surface. Thus, it is expected that NP nucleation takes place at these defects.26 To understand more about the structure and surface properties of the MUA-AgNPs-GO nanocomposite,
Fig. 1. (a) A schematic illustration of the MUA-AgNPs-GO nanocomposite synthesizes and (b) device structure of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction.
the SEM images were taken and the images are given in Fig.5a and b. As can be seen in the images, high density of the small sized AgNPs prevents the accumulation of the GO sheets and allows good coverage of the glass and the n-InP surfaces, respectively. The NPs located on the surface of the film were clearly observed in the SEM images. Temperature Dependent I–V Characteristics of the MUA-AgNPs-GO/n-InP Junction
The classical model of an ideal Schottky barrier diode considering thermionic emission theory pre-sents by the simple exponential form27
I¼ I0exp qV nkT 1 exp qV kT ; ð1Þ where I is the current flow through the metal– semiconductor junction, V is the voltage drop across the junction, q is the electronic charge, k is the Boltzmann constant, T is the absolute temperature, n is the ideality factor and Io is the saturation
current given by Io ¼ AAT2exp qUap kT : ð2Þ
In this equation, A is the effective diode area, A* is the effective Richardson constant as 9.4 A cm2K2 for n-InP, Uap is the zero bias
apparent barrier height. From Eq.2, ideality factor n can be written as follows:
n¼ q kT dV d ln I : ð3Þ
While n can be determined from the slope of the linear segment of the I–V plot, the intercept on the y-axis gives the saturation current. These values can be used to find out barrier height. Figure6
presents the forward and reverse bias I–V charac-teristics of the Ag/n-InP and the MUA-AgNPs-GO/ n-InP junctions at room temperature. As it is clear from the graphs, rectifying behavior is dramatically increased with the MUA-AgNPs-GO nanocomposite interlayer. Furthermore, the barrier height of the Ag/n-InP structure increased by using the interface between the metal and the semiconductor. The barrier height of the MUA-AgNPs-GO interlayer used junction increased from 0.420 eV to 0.630 eV due to influence of the space charge region of the Ag/ n-InP junction. This indicates that the barrier height can be controlled by choosing suitable mate-rials. The interface states on the semiconductor surface can lead to additional charges in the metal-semiconductor interface. By selection of the appro-priate interlayer, these charges at interface can be passivated and thus the barrier height of the junctions can be increased. The fact that the barrier height of the junction is controllable is one of the most important advantages of metal-semiconductor junctions. Furthermore, an optimized high barrier in the heterojunction results in a maximum average rectified current and a low saturation current. However, an uncontrollable oxide interfacial layer
in atmospheric conditions can change not only barrier height but also bias voltage sensitivity of the Schottky barrier height.28 Thus, MUA-AgNPs-GO nanocomposite layer coated onto the surface of the n-lnP inhibits the effect of atmospheric condi-tions and eliminates formation of the oxide
interlayer. The ideality factor n has values greater than unity and indicates that these junctions exhibit non-ideal diode behavior, which is probably due to the presence of an interfacial layer, interface states, series resistance, etc. At the same time, the ideality factor increases with decreasing
Fig. 3. (a) UV–Vis absorption spectra of the GO, the MUA-AgNPs and the MUA-AgNPs-GO dispersions in ethanol and (b) UV–Vis electronic absorption spectra of the MUA-AgNPs-GO nanocomposite film coated on the n-InP substrate.
temperature. This situation is attributed to the inhomogeneity of the barrier. However, this situa-tion is caused by tunneling and thermally assisted tunneling process in the heterojunction. These two processes are important at the low temperatures. Tunneling process assisted by thermal energy in the high electric field is ascribed to transitions of carriers penetrating the narrow potential wall between band states and traps. The captured and released charge carriers at the traps increase with increasing transitions. In this process, electrons in the conduction band cross through the narrow potential wall within the depletion region and afterwards thermalize to the trap levels. Thus, electron emission from filled traps occurs with moving electrons to the conduction band via ther-mally excitation for only a portion of the potential barrier and tunnelling through the remaining bar-rier, respectively.29
The linear and semi-logarithmic I–V plots of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction in the temperature range from 320 K to 80 K are
shown in Fig.7. The deviation from the linearity in the high forward voltage region is certainly due to the series resistance. The increase of the reverse current with the voltage is associated with the modulation of the MUA-AgNPs-GO Fermi energy by the applied voltage, which reduces the Schottky barrier for carrier injection from the MUA-AgNPs-GO to the n-InP.30,31As seen in Fig.7, the forward current is increased with increasing temperature due to increments of the carrier activation energy. Therefore, the specific on-resistance (RonS) of the
heterojunction has changed from 2.165 mX cm2 at 80 K to 0.853 mX cm2 at 320 K. Moreover, Figs.7
and 8 show that the maximum turn-on voltage is limited at around 0.489 V owing to the increase in forward current. Besides, the turn-on voltage is also limited due to increase of the leakage current flowing through the defects and the MUA-AgNPs-GO interlayer.31
Figure9 shows a linear correlation between the experimental effective barrier heights and the
Fig. 4. TEM images of (a) the AgNPs and (b) the MUA-AgNPs-GO nanocomposites.
Fig. 5. Representative SEM images of the as-synthesized MUA-AgNPs-GO nanocomposites at different scale bars (a) for 200 nm and (b) for 100 nm.
ideality factors. There are two linear regions having different slopes in this graph. From the extrapola-tion, the homogeneous barrier height was calculated
as 0.636 eV (in the range of 320–160 K) and 0.469 eV (in the range of 160–80 K). The barrier inhomogeneity causes a decrease in the effective
Fig. 6. Experimental semi-logarithmic I–V plots of the Ag/n-InP/Au-Ge and the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunctions at room temperature.
Fig. 7. The lin(I)–V characteristics of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device as a function of temperature. Inset shows log(I)–V characteristics of the heterojunction.
barrier height and increment in the n, especially at low temperatures.32When the barrier height is not homogeous, although the charge carriers cannot have adequate energy to pass high barrier height in
the lower temperature, but current transport take place at lower portions of barrier height.
The Norde functions enable calculation if both barrier height and series resistance value from the
Fig. 8. The turn-on voltage values of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device as a function of temperature.
Fig. 9. The barrier height versus ideality factor plot of the zero-bias apparent barrier heights and experimental ideality factors for the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device.
c q
where F(Vmin) is the minimum value of F(V) and
Vmin is the corresponding voltage. The barrier
height versus temperature plot obtained from Norde’s method for the Ag/MUA-AgNPs-GO/n-InP/ Au-Ge heterojunction is given in Fig.11. For real contacts (n > 1), the series resistance Rs can be
expressed as:
Rs¼
kT cð nÞ qImin
; ð6Þ
where Iminis the value of the forward current at the
voltage Vmin where the function F(V) exhibits a
minimum.
As seen in Figs.11 and 12, while barrier height values have increased with increasing temperature, series resistance values decreased. Because of the
Eq. 2can be written as35 ln I0
T2
¼ ln AAð Þ qUb
kT : ð7Þ
The activation energy ln(I0/T2) versus 1/nkT plot
(open triangles) was drawn and the effective barrier height value is obtained as 0.630 eV from this plot. The deviation in graphics at low temperatures is typically an indication of recombination in the depletion region, the presence of the spatially inhomogeneous barrier height and potential of the Ag/MUA-AgNPs-GO/n-InP/Ag-Ge heterojunction. Namely, current preferentially flows through lower barriers in the potential distribution.
The apparent barrier height and apparent ideal-ity factor,35respectively, are given by
Fig. 11. The barrier height versus temperature plots obtained from Norde functions for the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device.
Fig. 12. The series resistance versus temperature plots obtained from Norde functions for the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device.
Uap¼ Ub qr2s 2kT; ð8Þ 1 nap 1 ¼ q2þ qq3 2kT; ð9Þ
where Ub0 is the mean Schottky barrier height
(SBH) at zero bias (V = 0) and extrapolated towards zero T, rsis the standard deviation at zero bias and
Ub¼ Ub0þ q2V and standard deviation
rs¼ qs0þ q3V, where q2, q3 are voltage coefficients
that may depend on T. The experimental Uapversus
1/T and nap versus 1/T plots drawn by means of
experimental data obtained from Fig.14respond to two lines instead of single line. The experimental results extracted from both Eqs.8and9reveal the presence of two Gaussian distributions of potential barrier in the contact region. The values of the q2 are 0.021 in 160–320 K and 0.084 in 80–160 K range. The intercerpt and slope of straight line have given two sets of values of Ub0 and r0 as 0.572 eV
and 51 mV in the temperature range of 80–160 K and as 0.902 eV and 146 mV in the temperature range of 160–320 K, respectively. Although the value of Ub0is small, it is not insignificant compared
to the Schottky barrier inhomogeneity. In general, the low value of Ub0 signifies lower barrier
inhomo-geneity.27,33 Furthermore, considering the barrier height inhomogeneities, the effective Richardson plot is modified as follows:
ln I0 T2 q 2r2 s 2kT2 ¼ ln AAð Þ qUb kT: ð10Þ Figure15 shows the modified ln(I0/T2) (q2rs2/
2k2T2) versus 1/T plot. The calculations yielded zero bias mean barrier height Ub0of 0.572 eV (in the
range of 80–160 K) and 0.902 eV (in the range of 160–320 K). The intercepts at the ordinate give the Richardson constant A*of the n-lnP as experimental value of 3.82 A/cm2K2 in 80–160 K range and 6.55 A/cm2K2 in 160–320 K, which is close to the theoretical value of 9.4 Acm2K2. Hence, the barrier inhomogeneities at the metal–semiconductor inter-face for the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction can be explained by thermionic emission with the Gaussian distribution of barrier over the Schottky barrier heights.
Furthermore, the energy of interface states Ess
with respect to the bottom of the conduction band at the surface of the semiconductor (n-type) is given by27
EC ESS¼ qð/e ðV IRSÞÞ; ð11Þ
where the IRsterm is the voltage drop on the series
resistance. Depending on temperature change, the energy density distribution obtained from the I–V measurements and calculated Rs are shown in
Fig.16. It is clearly seen from Fig.16 that Nss has
an exponential growth from midgap of the InP towards to the bottom of the conduction band and
increases with decreasing temperature. This behav-ior of Nsscan be attributed to the restructuring and
rearrangement of the molecules of the metal–semi-conductor interface by the effect of temperature.36,37
CONCLUSIONS
In this paper, we emphasized the temperature dependence of forward and reverse bias I–V
Fig. 14. The apparent ideality factor (open squares) and the apparent barrier height (close triangles) versus 1/(2kT) curves of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device according to two Gaussian distributions.
Fig. 15. The ln(I0/T2) q2
rs2/2k2T2 versus 1/kT modified Richardson plots for the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction device
characteristics of the Ag/n-InP/Au-Ge junction mod-ified with uniform MUA-AgNPs-GO nanocomposite interlayers. The results of Raman, UV–Vis, TEM and SEM measurments showed that the MUA-AgNPs-GO nanocomposite prepared by the modified Hummer method was successfully synthesized and coated on the n-lnP substrate without morphologi-cal defects such as pinholes and voids. We also discussed the adaption of the thermionic emission theory to the Ag/n-InP junction with the Gaussian distribution at the changing temperature range. The calculated ideality factor and barrier height values showed strong temperature dependencies. The inhomogeneity of the barrier height potential at the interface is attributed to increased value of the ideality factor and decreased value of the barrier height with decreasing temperature. On the other hand, it is well-known that a heterojunction device is very sensible to modification of the metal–semi-conductor interface. It is clearly seen that the MUA-AgNPs-GO nanocomposites changed the barrier height and the Fermi level position. In the absence of pinning, changes of the MUA-AgNPs-GO Fermi level are strictly related to changes of the barrier height of the Ag/MUA-AgNPs-GO/n-InP/Au-Ge heterojunction. Therefore, with the MUA-AgNPs-GO nanocomposite interlayer, the working perfor-mance of the heterojunction improved due to an easily controllable feature of the barrier height.
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